Annular fluorescent lamps are generally manufactured by phosphor coating a straight glass envelope, baking the coating, heating the coated glass envelope to its softing temperature and then bending the coated envelope to a torus shape. If an adhesive is not used with the phosphor coating, the coating can flake or peel off during the bending operation. Adhesive materials that have been used in the past are alkali and alkaline earth borates, such as are disclosed in U.S. Pat. Nos. 2,905,572, 3,503,780 and 4,121,132. Zinc phosphate has also been used, as disclosed in U.S. Pat. No. 3,963,639.
A problem with the use of borates as the adhesive in phosphor coating is that it is difficult to remove all traces of water therefrom during lehring. Residual water in the coating is detrimental to lamp operations and lamp life. A problem with the use of zinc phosphate as the adhesive is that it does not have adequate adhesive power to prevent flaking during bending.